Protocol M97-720 is a Phase I/II, randomized, ABT-378 dose-blinded, multicenter study of ABT-378/ritonavir in combination with stavudine (d4T) and lamivudine (3TC) in otherwise healthy HIV-infected males and females. ABT-378 is a novel peptidomimetic HIV protease inhibitor with approximately 10-fold greater in vivo potency than ritonavir. The EC50 of ABT-378 in 50% human serum is approximately 0.1 micrometer. Against a panel of viral isolates demonstrating varying degrees of ritonavir resistance, ABT-378 demonstrated consistently superior activity compared with ritonavir. Co-administration of ABT-378 with ritonavir substantially improves the pharmacokinetic profile of the drug. The primary objective of this study is to assess the safety, tolerability, and antiviral activity of ABT-378/ritonavir when administered orally in antiretroviral naive HIV-infected patients. The study will also determine the steady-state pharmacokinetic profile and characterize HIV RNA decay profiles of ABT-378/ritonavir.